Engine systems utilize exhaust gas recirculation (EGR) to reduce engine generated NOx emissions, while at the same time increasing fuel economy by reducing engine pumping work. In EGR systems, exhaust gas from the exhaust manifold is typically routed to the intake manifold downstream of the engine throttle plate. However, degradation in EGR components can lead to degraded EGR operation. As such, various methods are available to monitor and diagnose such EGR degradation.
One such method is described in U.S. Pat. No. 4,715,348 (Kobayashi et al.). In this system, several embodiments are described for determining degradation of an EGR system. In one embodiment, variation in intake manifold pressure is used. In another embodiment, variation in the detected intake air amount is utilized. In still another embodiment, variation in temperature is utilized.
The inventors herein however, have recognized a disadvantage with such an approach. In particular, as emission requirements become greater and greater, simply using a single detection parameter can lead to inaccurate detection.
Another approach is described in EP 1239142 (Morikawa). In Morikawa, an engine controller feedback controls the open degree of an EGR valve such that an actual intake air amount matches a target intake air amount that corresponds to a running state of the engine. When feedback control is outside a relatively wide range, the controller alters the opening degree of the throttle in order to check for abnormalities in the EGR apparatus.
The inventors herein have also recognized a disadvantage with Morikawa. Specifically, simply using a feedback term in the feedback control of the EGR can lead to false abnormality detections since numerous other engine operating parameters can affect the EGR feedback control other than degradation of the EGR system.